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How to Megger a Motor

G.K. Bayne Updated February 21, 2017

Troubleshooting any electric motor can be accomplished with a simple volt ohm meter. Sometimes, though, the motor has more problems than a simple meter can detect. This is where the high-voltage Megger comes into play.

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Put on safety glasses. Disconnect and remove all power sources to the motor before attempting any type of repair. Double-check by using your volt ohm meter to ensure that all power is shut off.

Remove the four screws that hold the motor's wire connection cover in place, and set them aside. De-terminate, or unwire, the power wires from the motor wires. You may have to identify the pairs with labels or numbers so proper rotation can be maintained, especially in three-phase motors.

Attach one side of the Megger meter to one of the motor wires and the other to the motor ground or metal case. Turn on the meter or crank the handle. The meter's manufacturer should have reference readings for each meter. Generally, a reading above 20 to 30 megohms should be sufficient to run most motors. All new motors should read greater than 999 megohms; any reading less than this indicates winding deterioration to the insulation.

Test each motor winding in turn, and record each set of windings, per the megohm reading. If one of the legs has a very low reading, the motor should be replaced. This will also result in a high amperage draw or frequent blown fuses.

Tip

Always follow all manufacturer's safety precautions, as the Megger meter will produce high voltage at low amperage, generally around 1,000 volts. Consult all local electrical codes, which may require certain wiring practices and procedures.